The present invention relates to friction reducing polymers, and more particularly, to the use of complexing agents to improve the performance of anionic friction reducing polymers in water containing multivalent ions.
During the drilling, completion and stimulation of subterranean wells, treatment fluids are often pumped through tubular structures (e.g., pipes, coiled tubing, etc.). A considerable amount of energy may be lost due to turbulence in the treatment fluid. As a result of these energy losses, additional horsepower may be necessary to achieve the desired treatment. To reduce these energy losses, certain polymers (referred to herein as “friction reducing polymers”) have been included in these treatment fluids. In general, friction reducing polymers are high molecular weight polymers, such as those having a molecular weight of at least about 2,500,000. Typically, friction reducing polymers may be linear and flexible (e.g., persistence length<10 nm). One example of a suitable friction reducing polymer is a polymer comprising acrylamide and acrylic acid.
An example of a stimulation operation that may utilize friction reducing polymers is hydraulic fracturing. Hydraulic fracturing is a process commonly used to increase the flow of desirable fluids, such as oil and gas, from a portion of a subterranean formation. In hydraulic fracturing, a fracturing fluid may be introduced into the subterranean formation at or above a pressure sufficient to create or enhance one or more factures in the formation. Enhancing a fracture may include enlarging a pre-existing fracture in the formation. To reduce frictional energy losses within the fracturing fluid, friction reducing polymers may be included in the fracturing fluid. One type of hydraulic fracturing treatment that may utilize friction reducing polymers is commonly referred to as “high rate water fracturing” or “slick water fracturing.” As will be appreciated by those of ordinary skill in the art, fracturing fluids used in these high rate water fracturing systems are generally not gels. As used in this disclosure, the term “gel” refers to an interconnected assembly of macromolecules having temporary or permanent cross links and exhibiting an apparent yield point. As such, in high rate water fracturing, velocity rather than the fluid viscosity is relied on for proppant transport. Additionally, while fluids used in high rate water fracturing may contain a friction reducing polymer, the friction reducing polymer is generally included in the fracturing fluid in an amount sufficient to provide the desired friction reduction without gel formation. Gel formation would cause an undesirable increase in fluid viscosity that would, in return, result in increased horsepower requirements.